The field of this invention is pipeline positioning systems for use on a pipe laying vessel or the like.
Pipe laying vessels such as barges are utilized to lay a pipeline along the floor of an ocean or other body of water. Such pipeline barges generally include one or more welding stations for successively welding pipe lengths to the pipeline portion already formed. Each time new pipe lengths are welded onto the already formed pipeline portion, it is necessary to move the pipeline barge out from under the new pipeline portion to provide space on the barge for the addition of more pipe lengths. This is generally accomplished by an anchor pulling system mounted at the front of the barge for pulling the barge with respect to the pipeline portion already formed in order to provide space for the alignment and welding of new pipe lengths to the already formed pipeline portion In order to accomplish this relative movement betwen the pipeline portion already formed and the pipe laying barge or other vessel, it is necessary that some type of pipeline control mechanism be mounted on the barge to continually grip and allow movement of the pipeline portion with respect to the barge, particularly when the barge is moving forward as a result of the pulling forces exerted by the anchor pulling system or the like mounted at the bow of the barge. Such devices have been known in the art as "pipeline tensioners". This is somewhat of a misnomer since the pipeline tensioners are actually gripping devices which allow the pipeline portion to be fed off of the barge as the barge moves foward, the actual tension applied to the pipeline being the result of the pulling forces exerted on the barge by the anchor pulling system.
Generally, pipe lengths used to form underwater pipelines are weight-coated with concrete or the like. It is therefore necessary to provide gripping mechanisms for gripping and controlling the movement of the pipeline with respect to the barge which will not destroy this concrete coating. Another problem involves the addition of concrete coating in the joint area after the pipe lengths are welded together; for, in order to weld pipe lengths together on the barge, the end area of the pipe lengths are not weight-coated thus exposing the metal itself for welding purposes. The exposed joint area must be weight-coated after the pipe lengths have been welded together but before the pipe lengths are lowered into the water as part of the pipeline. Generally, an annular jacket is mounted about the exposed joint area after the welding and concetre is poured into the jacket. A pipeline tensioner or gripping device should be able to grip or hold the pipeline portion under tremendous forces without damaging the annular jacket which may be aluminum or the like, and the newly formed concrete layer therein. Such pipeline tensioners or gripping devices should also be able to grip and hold the position of the already formed pipeline portion extending from the barge in spite of changes in the diameter of the pipe lengths added to the pipeline.
One pipeline tensioning device known in the art involves the use of several sets of endless conveyor type track grippers which are mounted to receive the pipeline portion already formed between the grippers for allowing movement of the pipeline portion with respect to the barge or other vessel. The use of such endless conveyor type of gripping means do not solve all of the problems which have been mentioned. For example, it is necessary for such pipeline tensioners to grip and hold the pipeline portion already formed with tremendous gripping forces; this is due to the tremendous pulling forces exerted on the barge by the anchor pulling system and to the great weight of the already formed pipeline. The utilization of endless conveyor type of grippers causes some localization of the stress, thus making it more likely to destroy or otherwise damage part of the pipeline concrete coating. In addition, the concentration of stresses in particular areas by the endless conveyor type of track means increases the likelihood of damage to the annular jackets used to support freshly poured concrete.